The present invention relates to knee joint prostheses generally and to knee joint prostheses providing constrained articulation of the knee joint particularly.
A knee joint prosthesis typically comprises a femoral component (femoral) for replacing the surface of the femur at the knee joint, a tibial component (tibial) for replacing the corresponding tibial surface, and a patellar component for replacing the posterior surface of the patella. In a knee joint prosthesis intended to perform in the absence of functional cruciate ligaments, the femoral and tibial have complimentary geometries that limit the freedom of movement between the components to simulate the constraint and resulting motion normally provided by the ligaments.
A characteristic of normal knee motion is combined rotation and sliding of the femur with respect to the tibia during knee flexion. The rolling component of this motion results in the femur translating posteriorly with respect to the femur and is referred to as "rollback".
Another characteristic of normal knee motion is resistance to anterior displacement of the femur relative to the tibia in deep flexion. Such anterior displacement is referred to as anterior subluxation or anterior dislocation.
A representative and highly successful prior art knee prosthesis is taught in U.S. Pat. No. 4,298,992 granted to Burstein and Insall. That patent teaches a box-like recess between the condylar portions of the femoral having raised side walls extending from the anterior portion of the knee to the posterior portion of the knee and defining a box. A transverse convexly curved cam, integral with the box and forming a posterior boundary to the box, engages a concave surface on a spine extending from the tibial into the femoral box. To the extent that the spine extends into the femoral, bone must be resected to accommodate the spine and box.
This camming action forces the areas of contact between the tibial and femoral posteriorly, reproducing femoral rollback, to allow an increased range of flexion without impingement of the bone of the femur on the tibial component. The contact point between the cam and spine of prior art knees hits relatively high on the spine, thus requiring a large spine to resist the resulting shear and bending loads.